The present invention relates to a tissue expanding implant, and in particular, to a tissue expanding implant having a predetermined shape memory so that the implant will tend to reassume the predetermined shape after being collapsed and reinflated.
Tissue expanders are valuable in various surgical procedures where tissues must be expanded, particularly where tissues must be expanded to accommodate a prosthetic implant. Tissue expanders generally comprise flexible fluid-tight envelopes that are inserted subcutaneously into the portion of the patient's anatomy requiring augmentation. The tissue expander is inserted in a collapsed state. Various means are then employed to gradually inflate the tissue expander with a suitable fluid until the tissue expander, and thus the overlying tissue, expands to the desired extent. The means of inflating the tissue expander include tubes leading from the interior of the tissue expander to the exterior of the patient's body and the direct injection of fluid from a hypodermic needle through the patient's skin into the interior of the tissue expander. Once the tissue expander has been inflated to the desired extent, the tissue expander must normally be removed surgically from the patient and the actual prosthesis inserted.
Tissue expanders can be as simple as elastic balloons which expand when inflated in the same manner as balloons; i.e., substantially equally in all directions. It is desirable however that the tissue be expanded in accordance with the anatomical feature being replicated. A breast prosthesis implant, for example, should resemble a natural breast. The overlying tissue therefore is desirably expanded into complex shapes not easily achievable with simple elastic balloons. Numerous attempts have been made to achieve tissue expanders which are able to achieve complex shapes that more nearly replicate the shape of the desired tissue augmentation that is required to produce results that are anatomically satisfying.
One such solution is found in U.S. Pat. Nos. 5,035,249; 4,481,992 and 4,899,764, which disclose a tissue expander comprising a fluid tight envelope having portions with varying degrees of elasticity. The tissue expander is inflated and the varying degrees of elasticity control the amount of expansion of each portion and therefore the final shape of the tissue expander.
U.S. Pat. No. 4,574,780 discloses a tissue expander formed of a silicone elastomer such as Silastic.RTM. (Dow Corning). The tissue expander comprises a liquid impervious chamber with walls having a thickness that decreases from one end to the other. Expansion of the chamber by injection with fluid such as saline causes the thinner portions of the wall to expand to a greater extent that the thicker portions, thus allowing for differential expansion of the device.
In addition to the problem of achieving complex shapes of tissue augmentation suitable for implanted prostheses that replicate natural anatomy, there is the problem of injecting the fluid to gradually inflate the tissue expander. To avoid having tubes and the like penetrating the skin of the patient, it is desirable for the inflation to be accomplished by hypodermic needles through the skin of the patient and thence into the interior of the tissue expander. The elastic envelopes of tissue expanders, however, are typically not self-sealing and may be prone to undesirable leakage if penetrated by a hypodermic needle. The usual solution is to provide a self-sealing valve through which the hypodermic needle can penetrate without the risk of leakage.
U.S. Pat. No. 5,019,101 discloses a self-sealing valve for an implantable device. The valve includes a body with a channel which has an unevenly stressed section wherein the unevenly stressed section folds back to occlude the channel when a fill tube is withdrawn. The valve also includes a gel-filled chamber that occludes the end of the channel when the fill tube has been withdrawn.
U.S. Pat. No. 4,671,255 discloses a tissue expander comprising a silicone rubber cover attached to a base. A self-sealing injection reservoir is protected by a reinforcing member substantially more rigid that the expandable shell to prevent the folding of the expandable shell over the injection reservoir to prevent inadvertent puncturing of the expandable shell by a hypodermic needle.
It is known that latex rubber is self-sealing from a hypodermic needle puncture. This property has been employed in stoppers of drug vials. A hypodermic needle punctures the latex rubber stopper and withdraws fluids from the bottle. When the needle is withdrawn the puncture seals to avoid the loss of the remaining contents of the vial.
U.S. Pat. No. 5,005,951 avoids the problem of inflating the tissue expander. It discloses a self-inflating tissue expander comprising an envelope portion constructed of a material that is substantially impervious to body fluids. A window is attached to the envelope which is permeable to body fluids. An osmotic agent is disposed within the envelope and induces an osmotic differential across the window that draws body fluids into the envelope and thereby expands the envelope. A rigid base may be provided to limit expansion in particular directions.
The limitations of the prior art are overcome by the present invention as described below.